344 METALLURGY OF IRON AND STEEL. phosphorus, silicon, etc., but the carbon usually determines the class in which the material belongs. This selection of carbon as the one important variable arose from the fact that primitive Tubal Cains could produce a hard cutting instrument with no apparatus save # wrought-iron bar and a pile of charcoal; and the natural developments m manufacture have led to the conclusion that a given content of carbon will confer greater hardness and strength, with less accompanying brittleness, than any other element. - There are exceptions to this statement in hard steels made by manganese, chromium, or tungsten, but it is true in soft steel. It follows that no limit should be placed to the carbon allowed in structural material if a given tensile strength is specified. Every increment of carbon increases the hardness, the brittleness under shock, and the susceptibility to crack under sudden cooling and heating, while it reduces the elongation and reduction of area, but the strength must be bought at a certain cost, and this cost is less in •the case of carbon than with any other element. SEC. XYIIb.—Silicon.—The contradictory testimony concerning the effect of silicon on steel has been summarized by Prof. Howe.* He finds no proof that silicon has any bad effect upon the ductility or toughness of steel, and concludes that the bad quality of certain specimens is not necessarily due to the silicon content. A Bessemer ; steel with high silicon is sometimes produced by hot blowing, but it is wrong to compare such metal with the common product and ascribe all differences to the chemical formula, rather than to the circumstances which created that formula. J3ince the appearance of The Metallurgy, an able paper has been written by Hadfield,f who produced alloys with different contents of silicon by melting wrought-iron and ferro-silicon in crucibles. The metal was cast in ingots 2£ inches square, and these were reduced by forging to If inches square and rolled into bars 1^ inches tin diameter. In the list of analyses in the paper referred to, there are slight differences in the composition of drillings from different bars of the same ingot, but, in Table XVII-A, I have averaged the ' results of each cast so as to show the nature of the material under " investigation, and have given the physical results on the rolled bars in their natural state. * The Metallurgy of Steel, p. 36. t On Alloys of Iron and Silicon. Journal I. and S. I., Vol. II, 1889, p. 222.